The main reasons the Earth’s core is so hot is:
- Pressure – literally all the dirt and rock squeezed together. Thought to represent greater than 70% of heat production.
- Friction – different density and temperature layers grinding past each other. Thought to be 20%-25% of heat production.
- Radioactive decay – nuclear decomposition. Thought to be less than 2% of heat production.
- Chemical reaction and other processes. Basically everything else.
As other answerers have mentioned we are looking at the Earth 4.5 billion years later. So, yes the Earth is cooling, but at a progressively slower rate.
On the cooling:
- The Earth can really only lose heat by radiant methods.
- The Earth has an insulating layer, the crust, many miles thick, but only represents less than 0.1% of the thickness of the Earth. As the Earth continues to cool the crust thickens providing more insulation.
- The Earth’s atmosphere and oceans redistribute the heat that makes its way to the surface across the whole fairly evenly.
- Eventually, it will cool down to solid rock as you think of it today, but this is billions of years into the future.
Just as a parting thought, the vast majority of the heat we feel on the surface of the Earth comes from the sun, not the Earth. This isn’t a comment on intensity so much as the level of insulation between each source.
MKDesign 